The overall goal of this research is to elucidate the mechanisms by which oxidative stress mediates tumor promotion. The studies we conducted in the previous funding period indicate that overexpression of the mitochondrial antioxidant enzyme, manganese containing superoxide dismutase (MnSOD), suppresses the incidence and multiplicity of Papilloma in the DMBAJTPA multi-stage skin carcinogenesis model. Unexpectedly, reduction of MnSOD by heterozygous knockout of the MnSOD gene did not result in increased tumor incidence or multiplicity. The lack of increased susceptibility to DMBA/TPA-induced tumors in the MnSOD knockout mice is due to an increase in both apoptosis and proliferation after TPA treatment. The increased apoptosis in MnSOD-deficient mice is associated with increased p53 accumulation in the mitochondria.Based upon these novel findings, we propose to test the following four interrelated hypotheses and to determine how modulation of apoptosis and antioxidant levels, based upon the mechanistic insights learned from this study, may alter the outcome of tumor formation in a well-established skin carcinogenesis model. Specific aim 1: Apoptosis precedes proliferation in an oxidant-induced tumorigenesis model. Specific aim 2: Oxidative stress accumulating in mitochondria serves as a death signal, which induces p53 translocation to mitochondria. Specific aim 3: P53 translocation to mitochondria is an important event leading to amplification of the transcription-dependent apoptosis of p53. Specific aim 4: Increased p53 stability contributes significantly to increased p53 protein levels after TPA application. Specific aim 5: Determine if tumorigenesis is altered by redox-based intervention of apoptosis and proliferative pathways. MnSOD-transgenic and MnSOD-deficient mice, as well as wild-type mice in the same inbred background, will be used as in vivo models. The epidermal-derived JB6 variants will be used as in vitro models. The results of the proposed studies will provide information on the sequence of molecular events during an early stage of tumorigenesis, the mechanisms by which a previously unrecognized role of p53 may participate in the promotion stage of skin carcinogenesis, and a proof-of-principle for redox-mediated intervention of tumorigenesis.